Fluid pressure controlled actuating device



Nov. 20, 1951 A. J. BENT FLUID PRESSURE CONTROLLED ACTUATING DEVICE 2 SHEETS--SHEET 1 Filed Nov. 26, 1949 E s lfi Eur 21% mmAm -E w m g. mVE

INVENTOR. ARTHUR J. BENT ATTORNEY QQE m RV @W w. q.mwwvm vm waaaaaaazaa,

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FLUID PRESSURE CONTROLLED ACTUATING DEVICE Filed Nov. 26, 1949 2' SHEETS-SHEET 2 INVENTOR. ARTHUR J. BENT 97% zaah ATTORNEY Patented Nov. 20, 1951 FLUID PRESSURE CONTROLLED AGTUATING DEVICE Arthur J. Bent, Pittsburgh, Pa., assignor to Westinghouse Air Brake Company, a corporation of Pennsylvania Application November 26, 1949, Serial No. 129,651

This invention relates to a booster power device or fluid motor and more particularly to the type for controlling operation of devices such as the rudder or the ailerons of an airplane.

One object of the invention is to provide an improved fluid motor of the above type.

Another object of the invention is the provision of a relatively long stroke fluid motor wherein the quantity of fluid under pressure to be exhausted from one side and supplied to the opposite side of a reciprocating piston of the motor for effecting a short stroke of said piston, is materially less than in the conventional type of motor.

Still another object of the invention is the provision of a fluid motor such as above described,

in which, for a short stroke, the motor piston is more quickl operative in response to a change in pressure than the piston of the usual type of fluid motor.

Other objects and advantages will be apparent from the following more detailed description of the invention.

In the accompanying drawings; Fig. 1 is a plan view of a rudder booster system for an airplane constructed in accordance with one embodiment of the invention; Figs. 2 and 3, taken together, with the left hand end of Fig. 3 abutting the right hand end of Fig. 2, is a longitudinal sectional view of a fluid motor shown in elevation in Fig. 1, the major portion of Fig. 3 being drawn to a slightly larger scale than Fig. 2 to more clearly show certain details; Fig. 4 is a sectional view taken on the line 4-4 of Fig. 2; and Fig. 5 is a sectional view showing at an enlarged scale certain details of the power cylinder shown in Fig. 2.

Description As shown in Fig. 1 of the drawing, the reference numeral I designates the improved double acting fluid motor which includes a valve mechanism, carried in the interior of a piston rod 2 for controlling operation of said motor. The fluid motor I is adapted to be pivoted at one end by a pin 3 to any fixed member 4 such as a portion of the frame of an airplane. Extending from the opposite end of the fluid motor is the piston rod 2 which is connected to a rudder operating lever 5 pivoted on a rudder post it carried by the above mentioned frame. For providing manual control of the rudder, a pulley having spaced, parallel and integrally connected arms 8, formed integral therewith and disposed one at either side of lever 5. is also pivoted on the rudder post 6. As shown in Fig. 3 of the drawings, the arms 8 are connected to the above mentioned valve mech- 11 Claims. (01. 121-41) anism carried by the piston rod 2: by means of a pin 9 extending through each of said arms and one arm of a bifurcated yoke It forming a part of said mechanism, for operating said mechanism in a manner later to be described. Passing around the pulley I is a cable ll having the ends thereof connected to a foot operating lever l2 on opposite sides of a fulcrum pin l3 for said lever,

said pin also being carried by the frame of the airplane. Fastened to the ends of the foot operating lever l2 are pedals l4 and Is to provide means whereby a pilot may rock lever 52 in a clockwise or a counterclockwise direction about pin 13 by pushing on one or the other of the pedals.

The booster power cylinder l comprises a cylinder body It having a through bore i1 containing a main power piston H5 adapted for reciprocation within said bore. One end of the piston rod 2 is secured by screw-threaded engagement to the main power piston I8. The piston rod 2 is provided with means later to be described, for conveying fluid under pressure to and from one or the other of the faces of the piston It. Slidably mounted in the bore ll are two false pistons l9 and 20 one being disposed on each side of the main piston l8 and the false piston 20 being provided with a bore 2| through which projects the piston rod 2. The three pistons l8, I9 and 28 thus disposed in bore El form therein four pressure chambers 22, 23, 2d and 25, the chambers 22 and 23 being on the left and right hand sides, respectively, of the false piston 18, and chambers 24 and 25 being on the left and right hand sides, respectively, of the false piston 2t and the chambers 23 and 24 being at opposite sides of the piston l8. Each of the pistons l8, l9 and 20 is provided with a gasket ring 26 the peripheral surface of which is in sealing and sliding contact with the wall of bore l! to prevent leakage of fluid under pressure from the chamber located at one side of the respective piston to the chamber located at the opposite side thereof.

Secured at one end to the false piston H) by screw-threaded means is a stem 2'? extending through the chamber 22 to the exterior of the cylinder body 16 and having formed at the opposite end a collar 28 for engagement with a stop member 29 screw-threaded into a pressure head 30 secured to the left hand end of the cylinder body I 6 by any suitable means (not shown).

Disposed between the pressure head 3!] and the left hand face of false piston I9 is a coiled spring 3| for biasing said piston in the direction of the right hand to a position in which the collar 28 A passageway 32 formed'in the stem 21 is adapted to register with a passageway 33 formed' in the false piston I9, said passageways constitutvalve mechanism 34 hereinafter described, and atmosphere.

Secured at one end to the false piston 20 by means of screw-thread engagement is one end of a stem 55 extending through the chamber 25 to the exterior of the cylinder body it? and having formed at the opposite end a collar 56 for engagement with a stop member 31 screwthreaded into a pressure head 38 secured to the right hand end of the cylinder body i by any suitable means (not shown) Disposed between the pressure head 38 and the right hand face of false piston 20 is a coiled spring 39 for biasing said piston in the direction of the left hand to a position in which the collar 36 engages the stop 3l as shown in Fig. 2 of the drawings.

The stem 35 is provided with an axial bore 46 through which projects the piston rod 2 in sliding contact with said stem. A passageway 55 formed in the stem 55 in one side of the bore 40, as best seen in Fig. 4, isadapted to register with a passageway 42 formed in the false piston 20 said passageways constituting a fluid pressure communication between a valve mechanism 43, identical with the valve mechanism 35, and atmosphere.

The valve mechanisms 34 and 43 are identical but oppositely arranged and are carried by the respective false pistons I9 and 2t). Referring to Fig. 5 of the drawings, each of the false pistons is provided with a chamber M in which is situated a valve seat in the form of a collar 45 having a stem 46 slidably disposed in a bore 41 in the respective falsepiston, said stem extending into the respective chamber 23 or 25. The stem 46 is provided with a central passageway 48 extending through the seat on the collar 45 at one end of said stem and connected adjacent the opposite end to the peripheral surface of the stem through a series of ports 49. The peripheral surface of the stem 46 is in sealing and sliding contact with a gasket ring 58 clamped between the respective false piston is or 20 and a cap nut 5| screw-threaded into that face of the false piston adjacent the chamber 23 or 24. Disposed between the cap nut 51 and a collar 52, having screw-threaded engagement with the end of the stem 46 opposite the collarmunication between, the opposite sides of said,

valve. A'spring 51 is disposed in chamber 5 5 between supply valve 55 and a wall 58 of an ex-. haust valve cage 59 for urging said valve against.

its seat on collar 45.

The cage 59 is contained in chamber 22 or 25, as the case may be, and screw-threaded into the respective false piston. The valve cage 59 is provided on the side of the wall 58 opposite that 5 ing a fluid pressure communication between a engaged by the spring 5! with a chamber 60. Extending for a chosen distance into the chamber 60 is an annular valve seat 6| through which extends a bore 62 said bore being coaxial with the bore 4'! and openin into the chambers 44 and 50. Extending into the chamber 6!] and concentric with the annular valve seat BI is a shoulder 53 disposed from the wall 58 the same distance as the end of the annular valve seat 6|.

Disposed within the chamber 60 is a flat disc type exhaust valve 54 for cooperating centrally with the seat [5! to control communication between the respective chambers 44 and 60. The exhaust valve 64 has a plurality of spaced ports 55 for establishing a fluid pressure communication between opposite sides of said valve whenever said valve is moved out of seating engagement with the shoulder 53, and which communication is closed when said valve is in contact with said shoulder. The exhaust valve 64 is provided with a fluted stem 66 projecting through' the bore 52 into the chamber 54 and having its end dis-posed a chosen distance from the supply valve 55, when both valves are seated, to permit movement of said supply valve against the force of the spring 5? from its seat 45 said chosen distance without effecting the unseating of the exhaust valve 64.

In each of the false pistons 59 and 2D is a passageway 61 having a capacity substantially less than the capacity of passageways H and 13 and constituting a fluid pressure communication between the chamber 44 at the sprin side of the supply valve 55 and therespective chamber 22 or 25. v

The outer end of the chamber 60 is closed by a cap 58 having screw-threaded engagement with the exhaust valve cage 59, and supported by said cap and bearing against the valve 64 is a spring 59 for urging said valve to its seated position. A port in the cap 68 connects the chamber 50 at the spring side of the exhaust valve 64 to the respective chamber 22 or 25..

The piston rod 2 is provided with a central passageway H of greater flow capacity than the passageway 6'! in false piston I9 for a reason which hereinafter will be made apparent. The passageway ll registers at its end adjacent the main piston it with a central passageway 12 in the main piston I8, said passageways providing a communication through which fluid under pressure may be supplied to and released from the chamber 23. Concentric withthe passageway H in the piston rod 2 is a second passageway 13, also of greater flow capacity than the passageway 5'! in false piston 25, for a reason which hereinafter will be made apparent, one end of said passageway 13 terminating at the peripheral surface of said piston rod adjacent the right hand face of the main piston 18, as shown in Fig. 2 of the drawing. The passageway '53 provides a communication through which fluid under pressure may be supplied to and released from the chamber 24.

The end of piston rod 2 opposite the piston I8 is provided with a counterbore 14 (see Fig. 3) in which is disposed a valve mechanism F5 for controlling the supply and release of fluid under pressure to and from the passageways H and 13.

The valve mechanism 15 comprises a sleeve 16 slidably disposed in the counterbore 14. The end of the sleeve 16 opposite the passageways H and I3 is tapped to receive a cap 11 having a stem 18 to which the yoke I0 is secured as by a nut 19. The yoke I0 is bifurcated and its ends are connected respectively to arms by means of the pins 9, as hereinbefore mentioned, to effect movement of the sleeve I6 within the bore I4 in response to rocking of said arms about the rudder post 6.

The sleeve 16 is provided with an off-center bore 89 extending from near the left hand end of the sleeve to one end of a coaxially arranged and larger bore 8| also in said sleeve and which extends to the opposite end of the sleeve. Slidably mounted in the bore 80 is a plunger 82 one end of which extends beyond the end of the sleeve 76 and is screw-threadly connected to the piston rod 2. The plunger 82 is provided with a counterbore 33 open at its outer end to the passage 13 in the piston rod 2 and a plurality of ports 84 adjacent its right hand end connecting said counterbore to the peripheral surface of said plunger and thence to a passageway 85 extending through the sleeve 75 and opening into the bore 8|. Secured by screw-threads to the right hand end of the plunger 82 and slidably mounted within the bore 80 is an exhaust valve seat member 86 having a hollow tubular extension 81 projecting through the counterbore 83, the right hand end of the passageway I3, and into the central passageway 'II within the piston rod 2. A gasket ring 83 clamped between the piston rod 2 and a cap 89 secured thereto has sealing and sliding contact with the outer peripheral surface of the tubular extension 81 to prevent leakage of fluid under pressure between the passageways "H and I3. A plurality of ports 90 in the member 06 connect the interior of the tubular extension 8'! to the peripheral surface of member 86 and thence to a passageway 9I extending through the sleeve 16 and opening into the bore BI. A port 92 open at one end to the right hand end of the valve seat member 86 through a valve seat 93 formed thereon is open at the opposite end through a plurality of ports 94 to the peripheral surface of said member, said ports 94 being disposed in a position to register with con-- stantly registering ports 95 and 96 extending respectively, through the sleeve 16 and the piston rod 2 to form a communication between the port 92 and atmosphere. The seat 93 constitutes a seat for a disc type exhaust valve 91 which is adapted to control a fluid pressure communication between passage 85 and atmosphere through ports 92, 94, 95 and 96.

The valve mechanism 15 further comprises a plurality of members assembled in proper relationship in bore 8! by being slipped into the right hand end of said bore and held in place by a plug 98 closing the right hand end of the bore 8I said plug being secured in the sleeve 16 by the cap 11, as will now be described.

In the left hand end of the bore BI is a gasket ring 99 held in place by a plug I00 having screwthreaded engagement with the sleeve 16, said gasket ring having sealing and sliding contactwith the peripheral surface of the exhaust valve seat member 85.

Slipped into the bore 8! with the plug 98 removed, and in the order to be now described, are a plurality of members of the valve mech anism 15 the first of which is a stop IOI. The stop IOI comprises a center stop portion I02 adapted to be engaged by the exhaust valve 91 for limiting movement thereof in the direction of the left hand and two spacer elements I03 and E04 having screw-threaded engagement with opposite ends of the center portion I02, the spacer element I03 engaging the plug I00 while the plug 98.

the spacer element I04 engages one side of a partition element I05. The spacer element I03 has adjacent its end opposite the center stop portion I02 a row of circumferentially arranged ports I06 in permanent communication with the ports in sleeve 16. The spacer element I04 has two axially spaced circumferentially arranged rows of ports I06a, one of which rows registers with ports extending through. the center stop portion I02 to a chamber I01 surrounding the seat 93 for the exhaust valve 91 while the other row of ports open to a chamber I08 at the opposite side of said valve. The two rows of ports [06a are open to each other through annular clearance space provided between the Wall of bore BI and the outer surface of spacer element I04.

Clamped between the center stop portion I 92 and the spacer elements we and I94 respectively, are two gasket rings its and IIIO having sealing and sliding contact with, respectively, the peripheral surface of the exhaust valve seat member 85, and the wall of the bore 8|.

The side of the partition element I opposite that engaged by the spacer element I04 is engaged by one end of a spacer sleeve III having adjacent each end a row of circumferentially arranged ports I I2 to provide a fluid pressure communication between portions of a chamoer H3 within and encircling said sleeve and which chamber is open to one end of a passageway II4 extending through the sleeve l6 and opening at its opposite end to a chamber II5 formed in the cap H at the right hand side of The opposite end of the sleeve III engages one side of a second partition element i It, the opposite side of which is engaged by one end of a spacer element Ill constituting a part of a second stop H3 comprising the spacer element Ill, a center stop portion II9 for limiting the movement of a second disc type exhaust valve I20 in the direction of the right hand, and a second spacer element I25, said spacer elements II! and I2! having screw-threaded engagement with opposite ends of the center portion H9. The spacer element 52! has adjacent its end opposite the center stop portion II9 a pair of oppositely arranged elongated slots I22 in permanent registry with a pair of similarly arranged slots H3 in sleeve it which latter slots open to atmosphere exteriorly of the piston rod 2. The spacer element Ill has two axially spaced circumferentially arranged rows of ports I24, one of which rows is open through ports extending through the center stop portion II9 to a chamber 52s surrounding a seat E29 formed on a valve seat member 527 which is slidably within a core Q29 extending through the center stop portion M9 and spacer element I2I, while the other row of ports to a chamber I29 at the opposite side of the valve I29. The two rows of ports 124 are open to each other through annular clearance space provided between the wall of bore 8! and the outer surface of the spacer element I I1.

Ciamped between the center stop portion II9 the spacer elements ii! and I2! respectively, are two gasket rings 53d and IZH. The gasket ring has sealing and sliding contact with the peripheral surface the exhaust valve seat member I21, and the gasket ring :3! has sea1- ing contact with the wall of bore BI.

A port 532 open at one end to the left hand end of the valve seat member :2? through the valve seat I25. formed thereon is. open at. the

7 opposite end to atmosphere through ports I22 and I23. The seat I25 constitutes a seat for the disc type exhaust valve I25.

The partition elements I55 and H6 are each provided with an axial bore I33 having atthe end adjacent the chamber II3 a valve seat I34. Disposed in the chamber II3 are two identical but'oppositely arranged valves I35 and I36 for cooperation with the seats I34 on partition elements I55 and H5, respectively. The valves I35 and I35 are provided with fluted stems I3! and I33 respectively, said stems extending through the'bor'es I33 and into engagement with the exhaust valves 9! and; I25 respectively. Disposed in'the chamber [I3 and, supported by and bearing against the supply valves I35 and I36 is a spring I39 for urging each of said valves to its seated position.

After all of the elements of the valve mechanism I5 are assembled in the bores 35 and 8| in the order described in the preceding paragraphs, the right hand end of the bore 8I is closed by the plug 98 which engages the right hand end of the spacer element IZI. The plug 98 is provided at its right hand side with a shoulder I 55 and a threaded portion I II. A gasket ring- I42 rests against the shoulder I45 and is locked in place by a lock nut I ls which engages a shoulder I 44' formed on the sleeve I5 at the right hand end of the bore 8|. The gasket ring I52 engages the wall of bore 3I to prevent leakage of fluid under pressure from the chamber H5 to atmosphere through bores I22 and I23. With the plug 98 and lock nut I43 occupying the above described positions, the cap I7 is screwed into the right hand end of the sleeve I5 until the left hand end thereof engages the lock nut I 43 for thereby holding said lock nut and the other parts assembled in the bore III in the relation above described. With the valve mechanism I5 thus assembled, the chamber I I5 is open through a passageway I55 in the stem I8 of the cap Ill and a flexible hose I46 to a fluid pressure storage reservoir Id? (Fig. 1) which may be charged with fluid under pressure by any suitable means (not shown).

Secured to the right hand end of the exhaust valve seat member I21 are two pins I48 and I59 extending respectively, in opposite directions from said valve seat member through the slots I22 and I23 in the spacer member I2I and sleeve I6 and connected at their outer ends to a spindle member I 50 having screw-threaded engagement with the right hand end of the piston rod 2. A look nut I5I having screw-threaded engagement with the piston rod 2 is provided for engagement with the spindle member I50 to lock said spindle member in an adjusted position with respect to the piston rod 2.

The spindle member I5 is provided with oppositely arranged spindles I52 and I53 connected to the bifurcated end of the rudder operating lever 5.

Slidably extending through a bore I54 in the yoke I is a cap screw I55 adjustably connected to the spindle member I by screw-threaded engagement for limiting movement of the yoke I5 and the sleeve I5 in the direction of the right hand with respect to the piston rod 2 and the spindle member I50 to a degree just sufficient to effect full opening of the supply valve I33 from its seat I34. A look nut I 5? having screw-threaded engagement with the cap screw I55 is pro vided for contact with the spindle member I55 8 to lock said cap screw in an adjusted position with respect to the spindle member I55. 7

Adjustably connected by screw threads to the yoke I0 is a cap screw I having a stem I59 eX- tending into the path of movement of the spindle member I5 for engagement thereby for limiting movement of the yoke Ill and the sleeve I5 in the direction of the left hand with respect to the piston rod 2 and the spindle memberI55 to a degree just sufficient to efl'ect full opening of the supply valve I35. A lock nut I5 having screwthreaded engagement with the cap screw I58 is provided for contact with the yoke II) to lock said cap screw in an adjusted position with respect to the yoke I0.

In order to prevent leakage of fluid under pressure between the chamber I I3 and the chambers I08 and I29, the respective partition elements I 55 and I I6 are each provided with a resilient gasket ring I5I having sealing contact with the wall of the bore 8 I. In order to prevent leakage of fluid under pressure from the passageway 55 to the passageway 9I and bore I4, two gasket rings I62 are provided, said two rings being carried adjacent the left hand end'of the sleeve I5 and having sealing andsliding contact with the plunger 82.-

Operation.

With the storage reservoir Itl charged with fluid under pressure, fluid will flow therefrom to the chamber H5 and thence through the passageway II4 to the chamber H3 wherein it acts on the supply valves I35, I35 to urge them against their seats.

If it is now desired to rock the rudder operating lever 5 (Fig. 1) in a clockwise direction, the pilot will push on the pedal I I to rock the operating lever I2 in a clockwise direction about the pin I3. The clockwise rocking of the lever I2 about pin I3 will eifect, through the cable I I and the pulley I, clockwise rocking of the arm 8, which, by reason of its connection to the yoke I5 through the pins 3, Will exert a push on said yoke and the sleeve I5 and thereby the partition elements I05 and H6 urging same (Fig. 3) in the direction of the left hand.

The area of the supply valve I35 subjected to pressure of fifteen hundred pounds per square inch present in the chamber II3 is such that as long as the resistance to movement of the power piston I8 and rudder operating lever 5 does not Xceed the force pressing said valve against its seat said valve will move in the direction of the left hand simultaneously with the partition element I05. This movement of the supply valve I35 therefore will be transmitted through the fluted stem of said valve, the exhaust valve 91 and the-exhaust valve seat member to the piston rod 2 and the power'piston I8 which will be moved in the direction of the left hand and act ate the spindle member I58 to correspondingly position the rudder lever 5. While the plane is in flight the pressure of fluid pressing the supply valve I35 to its seat will not permit manual operation of the rudder lever 5 as just described through more than perhaps one degree from its neutral position at which time the resistance to rudder movement will become so great that by manual pressure on pedal I 4 the partition element I05 will be moved away from the supplv valve I35 and the exhaust valve 525 will be opened by center stop portion I I9 to permit power opera tion of the rudder, as will be later described. If the pressure of supply fluid in chamber I It should be less than the intended fifteen hundred pounds, the supply valve I35 will be thus unseated with less manual pressure on the pedal I I than when said chamber is charged to the full fifteen nun dred pounds, as will be apparent.

On the other hand, if the plane is on the ground and the operator is testing movement of the rudder when its resistance to movement is at a minimum, the pressure of fluid in chamber I I3 pressing the supply valve I35 to its seat will be sufficient to cause greater manual movement of the rudder lever 5 clockwise than when in flight. Under such a condition it will be noted that with both supply valves I35 and I35 seated, as well as both exhaust valves 9?, I25, manual movement of the power piston it toward the left hand will act to compress fluid in chamber 23 and reduce pressure of fluid in chamber 25 thereby creating a force, equal to the differential cc-- tween such pressures acting on the opposite sides of the power piston I8, which opposes the manual pressure on pedal I4. When this force becomes equal to or exceeds the force developed by fluid under pressure in chamber I I3 pressing the supply valve I55 to its scat then said valve will be come uuseated as well as the exhaust valve iEQ to permit pressure of fluid in chamber 25 to be increased by flow from chamber H5 past the supply valve I55 and to also permit reducing pressure of fluid in chamber 23 by flow past the exhaust valve I25, whereby the force on piston I5 opposing manual movement thereof b pedal I I will be so reduced as to permit further turning of the rudder lever 5 by said pedal.

, If manual movement of the power piston as just described, is sufficient it will engage the collar 52 of the valve mechanism as and force said collar and the stem 45 in the direction of the left hand until the spring 53 is compressed solid after which the false piston I5 will be moved in the direction of the left hand against the force of the spring SI.

At any time when the resistance to clockwise movement of the rudder operating lever 5 requires a force greater than applied manually to pedal I4, the yoke IE} will be moved in the direction of the left hand which will move the sleeve I5 relative to the piston rod 2 and thereby move the exhaust valve I25 away from its seat and at the same time move the partition element I95, away from the supply valve I55. When the supply valve seat I35 is thus moved away from the supply valve I35, fluid under pressure from the reservoir I ll, with said reservoir charged, present in the chamber H5 will flow past said valve and along the fluted stem thereof to chamber I fiiiand then through ports 555a, passageway 85, ports 85, counter-bore 83, and passageway IS in the piston rod 2 to the chamber 24 at the right hand side of piston I5; chamber 23 at the opposite side of said piston being vented at this time past theunseated exhaust valve I20.

As fluid under pressure is thus initially supplied to'the chamber 25 it will flow through ports 49 (see Fig. 5) of the valve mechanism 43', the

passageway 58 and against the left hand face of the flatdisc typesupply valve 55. The'fiuidu'nder pressure acting on the left hand face or the valve 55 will move said valve against the force of the spring 57 out of contact with the valve seat 45 to permit flow of fluid under pressure through the ports 56 in the valve 55 and thence through the passageway M to the chamber 25.

Fluid under pressure will continue to thus flow to the chambers 24 and 25 until the pressure in chamber 24 acting on the right hand face of the main power piston I8 is great enough to over-' come the resistance to movement of the rudder lever 5 at which time the piston I8 and piston rod 2 will be moved in the direction of the left hand to thereby rock the rudder lever 5 in a clockwise direction about the rudder post 5.

Movement of the piston rod 2 in the direction of the left hand carries with it the exhaust valve seat members and I2]. As the member 86 moves in the direction of the left hand, the force of spring- 63.? acting on the supply valve I55 will, through the fluted stem of said valve, maintain the exhaust valve ill in contact with the valve seat 53 and move the supply valve I35 in the direction of the left hand. Assuming that less than full movement of pedal l4, sleeve "I8 and partition I05 is effected, the supply valve I35 will thus continue to move until it seats on the scat I54 which terminates further flow of fluid under pressure from the storage reservoir I41 to the chambers 24 and 25, whereupon movement of the piston I8, piston rod 2 and rudder operating lever 5 will cease with the rudder operating lever 5 in a position corresponding to the position of the pedal I4, it being noted that the exhaust valve I25 will also reseat at the same time as the supply valve I35.

If the pilot pushes on the pedal I4 to cause a further clockwise rocking of the operating lever I2 about the pin I3, the arm 8 will be further rocked clockwise to move the sleeve I5 further in the direction of the left hand to again open the supply valve I35 to supply fluid under pressure from the storage reservoir I4! to the chambers 24 and 25 and to at the same time open the exhaust valve I25 to vent chamber 23. Fluid under pressure will thus flow from the storage reservoir I41 to the chambers 24 and 25 until the pressure in the chamber 24 is great enough to overcome the resistance to movement of the rudder lever 5 and move the piston I8 and piston rod 2 further in the direction of the left hand to a new position corresponding to that of pedal I 4 and in which the supply valve I35 and exhaust valve I20 are again closed in the manner herein-' before described.

It is therefore seen that the pilot, by pushing on the pedal M, may supply fluid under pressure to the chambers 24 and 25 until the chamber 24 is charged to any degree of pressure necessary to effect positioning of the rudder lever 5 to correspond to that of said pedal.

If the pilot desires to move the rudder lever 5 in a counterclockwise direction, he will push on the pedal I5 to rock the operating lever I2 in a counterclockwise direction about the pin I3. The counterclockwise rocking of the lever I2 about pin I3 will eirect, through the cable I I and the pulley I, counterclockwise rocking of the arm 8' which, by reason of its connection to the yoke I0 through the pins 9, will pull said yoke and the sleeve 15 in the direction of the right hand.

If the force required to turn the rudder lever 5 counterclockwise does not exceed that applied to the pedal I5 then counterclockwise rotation will occur solely by manual pressure on pedal 85 the same as clockwise rotation resulted from manual pressure on pedal I4, as above described, but if the resistance to counterclockwise movement of the rudder lever requires a greater force, the manual movement of sleeve I6 in the direction of the right hand will be relative to piston rod 2 and will move the seat I34 away from the supply valve I35 whereupon fluid under pressure present in the chamber H3 will flow past said valve, along the fluted stem of said valve, through ports i224, passageway 9|, ports 95 in the tubular extension 81 and central passageways Z! and E2 to the chamber 23.

As the sleeve 73 moves in the direction of the right hand relative to piston rod 2 it carries with it the stop till, as hereinbefore mentioned, and thereby lifts the exhaust valve 9'! from its seat 93 on the right hand end of the exhaust valve seat member 86. As the exhaust valve 91 is moved away from the seat 93 the chamber 24 is opened to atmosphere through the passageway 23, counterbore 53, ports 84, the passageway 85, ports its past the open exhaust valve 9! and through the ports 92, 94 in the exhaust valve seat member 85, port I55 in the spacer element I53, and ports 95 and 26 in sleeve l6 and piston rod 2 respectively thus venting fluid under pressure from the chamber 24 at the same time that fluid under pressure is being supplied to the chamber 23.

It is desired to point out that fluid under pressure is vented only from the chamber 24 since the spring 5! will maintain the valve 55 seated and prevent back flow of fluid under pressure from the chamber 25 to chamber 24.

The fluid under pressure supplied to the chamber 23 flows through the ports 49 (see Fig; 5) of the valve mechanism 34, passageway 48 and against the right hand face of the valve 55. The fluid under pressure acting on the right hand face of the valve 55 moves said valve against the force of the spring 5i out of contact with the valve seat 45 to permit flow through the ports 55 in said valve and thence through the passageway 6? to the chamber 22.

Fluid under pressure will thus continue to flow to the chambers 22 and 23 and from the chamber 24 in the manner described in the foregoing paragraphs until the differential in pressures acting across the main piston I3 is great enough to overcome the resistance to movement of the rudder operating lever 5 at which time the piston is and piston rod 2 will be moved in the direction of the right hand to thereby rock the rudder lever 5 in a counterclockwise direction about the rudder post 6.

Movement of the piston rod 2 in the direction of the right hand carries with it through the pins 548 and Me the exhaust valve seat member i271. As the member l2! moves in the direction of the right hand, the force of the spring I39 acting on the supply valve E35 will, through the fluted stem of said valve, maintain the exhaust valve H25 in contact with the valve seat I26 and move the supply valve I35 in the direction of the right hand. Assuming that less than full movement of pedal E5, sleeve 18 and partition H6 is efiected, the supply valve I36 will thus continue to move until it seats on the seat I34 in a position of piston is corresponding to that of the pedal I2 and which terminates further flow of fluid under pressure from the storage reseri operating lever 5 in a position corresponding to the position of pedal l5.

Movement of the piston rod 2 in the direction of the right hand also carries with it the exhaust valve seat member 86 which will engage the exhaust valve 9'3 at substantially the same time that the supply valve I36 engages its seat I34 thus simultaneously terminating exhaust of fluid under pressure from the chamber 24 and the supply of fluid under pressure to the chambers 22 and 23.

If the pilot pushes on the pedal ii to cause further counterclockwise rocking of the operating lever I2 about the pin I3, the arm 8 will be further rocked counterclockwise to move the sleeve 16 in the direction of the right hand to again open the supply valve I36 to supply fluid under pressure from the storage reservoir I47 to the chambers 22 and 23, and the exhaust valve 9! will be opened to release fluid under pressure from the chamber 24, and such supply and release will continue until the pressure differential across the main power piston I8 is great enough to overcome the resistance to movement of the rudder lever 5 and move the piston I8 and the piston rod 2 in the direction of the right hand until the valves I36 and 91 are closed in the manner hereinbefore described.

It is therefore seen from the foregoing description that the pilot, by pushing on the pedal I5, may supply fluid under pressure to chambers 22 and 23 until the chamber 23 is charged to the degree of pressure necessary to efiect desired positioning of the rudder operating lever 5.

In order to return the rudder lever 5 to any desired position in a clockwise direction of movement, the pilot will push on pedal M to thereby actuate the sleeve 75 to open the supply valve I35 and exhaust valve I25 whereupon fluid under pressure will be supplied to chamber 24 and at the same time released from chamber 23 until a sufiicient difierential in pressure is established on opposite sides of piston I8 to move the rudder lever 5 clockwise to the position corresponding to that of said pedal. 7

It will now be seen that by operation of pedals I4 or I5 any desired position of the rudder lever 5 will be effected by pressure of fluid acting on the power piston I8 when the resistance to movement of said lever is in excess of that required to permit manual movement of said lever as hereinbefore described.

In the power operation above described, that is, movement of piston I8 between positions in which it will just contact collars 52 on the valve mechanisms 34 and 43, respectively, the chambers 22 and 25 at the outer faces of the false pistons I9 and 20 will be only once charged with fluid under pressure by flow of fluid under pressure from chambers 23, 24, respectively, as upon the initial operation of pedals l4 and I5, after which they will remain charged due to the closed check valves during operation of the piston I8 between said positions, whereby with the false pistons I6, I! in their innermost positions pressure changes need be effected only in the relatively small chambers at opposite sides of piston I8 to effect positioning of the rudder lever 5, thereby conserving fluid under pressure and obtaim'ng quicker response of said piston to operation of the pedals i 4 and I5 than if chambers 22 and 25 were a part of and constantly open to chambers 23 and 24, respectively, as in conventional motors.

The power operation above described is, it will be noted, limited to movement of the power piston 58 to positions between the false pistons 19, 293 when in their inner positions, in which they are shown in the drawing, such movement of said power piston being adequate to provide such adjustment of a rudder as will usually be required during normal flight of a plane.

Now assume that the operator desires to move the rudder lever from a neutral position, in which said lever and piston I8 are shown in the drawing, in a clockwise direction to an extent greater than for normal flight and requiring greater movement of piston I8 than just into contact with collar 52 of the valve mechanism tit. To thus position the rudder lever 5, the pilot will push pedal I4 to, the position desired for lever 5 and thereby open the supply valve E35 to cause fluid under pressure to be supplied to the chambers 24 and and at the same time open the exhaust valve [2?] for venting fluid under pressure from the chamber 23 in the manner hereinbefore described. lhe piston 13 will then move in the direction of the left hand until the left hand face of said piston engages the collar 52 of the valve mechanism 34. Upon the left hand face of the piston I8 engaging the collar 52 of the, valve mechanism. 34, continued movement of the piston IS in the direction of the left hand will displace the stem db and valve 55 of said valve mechanism in the direction of the left hand until the spring 53 is compressed solid which not only opens the valve 55 but also through the fluted stem E36 lifts the exhaust valve 64, from its seat ti against the force of spring 39. Fluid under pressure will thereby be vented from chamber 22 through the passageway bl, bore 62, past the unseated exhaust valve: Gd, chamber Bil, and passageways 33 and to atmosphere whereby the false piston 19 will move with piston l8, resisted only by spring iii, until piston it obtains the position corresponding to that of pedal l4. Upon the pilot ceasing to press on the pedal l t, the piston it, piston rod 2, exhaust valve seat member and exhaust valve 9: will continue to move in the direction of the left hand until spring I39 seats supply valve I 35 to cut on the supply of fluid under pressure to the chamber 25 and the exhaust valve lfil seats to prevent further release of fluid under pressure from chamber 23 whereupon the pistons 18 and is and rudder operating lever 5 will come to rest in a position in which said piston occupies a position at the left of the position in which the false piston iii is shown in Fig. 2 and the rudder operating lever Pi will occupy an angular position to the left of the position shown in Fig. l of the drawings. With the piston is at rest, fluid under pressure in chamber 22 will continue to flow to atmosphere past the open valve 64 in valve device and through passageways 33 and 32 until the pressure in chamber 22 is reduced below that in chamber 23 sufiiciently to permit the pressure in chamber 23 to move the piston It? further in the direction of the left hand until the spring til seats exhaust valve 6 1 and the pressure retained in chamber 22 together with the force of spring 3| balances the pressure in chamber 23. 7

Now let it be assumed thatwith the pistons $3 and is and the rudder operating lever 5 occupying the positions just described, the pilot desires to rock said lever in a counterclockwise direction about rudder post 6. To rock the rudder operating lever 5 counterclockwise, the pilot 14 will push on the pedal :5 which will cause fluid under pressure to be supplied to the chamber 23 and vented from the chamber 24 in the manner hereinbefore explained.

Fluid under pressure thus supplied to chamber 23 will flow through ports 49 and passageway 48 in stem lfi of valve mechanism 34 and against the right hand face of the supply valve 55 over an area equal to the area of the passageway 48. When the pressure in chamber 23 and passageway 48 is thus raised sufficiently to overcome the force of spring 57 on the supply valve 55 said valve will move out of engagement with collar 35 whereupon fluid under pressure will again flow to chamber 22 to charge same to substantially the pressure in chamber 23. When the increase in pressure in chamber 23 and the decrease in pressure in chamber 24 has continued sufficiently to establish a differential in pressures on the oppositesides of piston I8 sufficient to overcome the resistance to movement of the rudder operating lever the piston i8 will move in the direction of the right hand to rock the rudder lever 5 counterclockwise.

As the main piston is thus moves in the direction of the right hand, the spring 3| will move the false piston is in the same direction, the false piston i9 following the main piston l8 until the collar 23 engages the stop member 29 which will prevent further movement of the false pis ton i9, but during such movement fluid under pressure will continue to flowto chamber 22 past the valve 55 to maintain such pressure equal substantially to that-in chamber 23.

The distance the piston I3 is moved in the direction of the right hand will of course depend on the displacement of the pedal l5 by the pilot, the rudder operating lever 5 being always moved to a position corresponding to the position of the pedal l5. If the pilot desired. that piston l3 be moved further in the direction to the right than just into engagement with collar 52 of valve mechanism 43, then by continuing to move pedal It the piston is will correspondingly move and during such movement operate the valve mechanism 43 to vent fluid under pressure from the chamber 25 so that the false piston 26 will move in the same direction, until piston l8 obtains the position corresponding to that of pedal IE. to vent the chamber 25 is identical with the operation of the valve mechanism 34 to vent the chamber 22 as hereinbefore described.

Upon the pilot discontinuing to push on the pedal l5, the valve mechanism 15 will operate in the manner hereinbefore described to cut off the supply of fluid under pressure to the chambers 22 and 23 and the venting of fluid under pressure from the chamber 25. Thus, by pushing the pedal it to any selected position, the pilot may effect movement of the piston IE! to any corresponding position between the positionwhich piston l8 would occupy when the false piston I9 is moved to its final position in the direction of the left hand and the position which piston [8 would occupy when the false piston 20 is moved to its final position in the direction of the right hand.

If prior to charging the storage reservoir I41 and chamber H3 with fluid under pressure, the operator desires to test, on the ground, turning of the rudder lever 5 clockwise from neutral position and pushes on pedal Hi to accomplish same, under which condition the supply valve H35 will be pressed to its seat onlyby the slight force of The operation of the valve mechanism d3 bias spring !39, .the spacer elements J2! and IE3 and partition elements H6 and H35 will move with the yoke if! relative to the exhaust valve seat members 12f and 8t permitting the spacer element E2! to open the exhaust valve I20 and the seat member 86 to open the supply valve l35. With the exhaust valve I20 open chamber 23 will be open to atmosphere while with the supply valve I35 open chamber24 will be opened to chamber H3 and thereby to the fluid pressure storage reservoir M? at atmospheric pressure. Continued pressing against pedal M will thus cause the piston l8 to be moved, with substantial equal fluid pressures acting on opposite sides, to move the rudder lever 55 and correspondingly position the planes rudder, the exhaust valve lid and supply valve l35 remaining open during such continued operation. In contrast to the illustration, the volume of the supply reservoir Ml will be relatively great as compared to the volume of chamber 2 1 so that the reduction in pressure in said chamber resulting from movement of piston l8 toward the left hand will be substantially negligible with respect to manual effort applied to pedal It. If movement of the pedal I4 is such as to cause piston it to engage collar 52 of valve mechanism 31% and open the respective exhaust valve 6 3, chamber 22 will remain at atmospheric pressure during further movement of said piston whereby only the light pressure of spring 3! will oppose further movement of pedal 14.

On the other hand, if the pedal I is operated to turn the rudder lever 5 counterclockwise, the exhaust valve 9? and supply valve 836 will be opened, and finally valve 54 inthe valve mechanism 43 if pedal movement is sufficient, to permit manual movement of piston 18 the same as just described in connection with manual turning of rudder lever 5 in a clockwise direction.

Summary It will now be seen that by the use of the false pistons l9 and 26 in a relatively long stroke motor the amount of fluid under pressure required for effecting chosen shorter stroking of the motor is less than in conventional motors since after the initial operation of the motor the necessary pressure variations need only be effected in the relative small chambers 23 and M, but when longer stroking of the motor is desired the pistons 59 and 28 will be displaced as necessary.

Having now described the invention, what I claim as new and desire to secure by Letters Patent, is:

1. A fluid motor comprising a casing having a bore, a double acting power piston slidably mounted in said bore, two false pistons slidably mounted in said bore, one at either side of said power piston, and cooperative with said power piston to form two main pressure chambers one at either side of said power piston, an auxiliary pressure chamber at the opposite side of each false piston, means acting on each false piston urging it toward said power piston to a position providing for limited movement of said power piston between the two false pistons, means for conveying fluid under pressure to the main pressure chamber at either side of said power piston and for at the same t'nne venting the main pressure chamber at the opposite side, one way flow means for conveying fluid under pressure to each auxiliary chamber upon supply of fluid under pressure to the adjacent main pressure chamber, and means operative by said power piston upon movement into substantial contact with either of said false pistons to release fluid under pressure from the auxiliary pressure cham-' her at the opposite. side of the respective false piston. i

2. A fluid motor comprising a casing having a bore, a double acting power piston slidably mounted in said bore, two false pistons slidably mounted in said bore, one at either side of said power piston, and cooperative with said power piston to form two main pressure chambers one at either side of said power piston, an auxiliary pressure chamber at the opposite side of each false piston, means acting on each false piston urging it toward said power piston to a position providing for limited movement of said power piston between the two false pistons, means for conveying fluid under pressure to the main pressure chamber at either side of said power piston and for at the same time venting the main pressure chamber at the opposite side, and valve means carried by each of said false pistons comprising a check valve in a communication providing for flow of fluid under pressure only from the main pressure chamber at one side of the respective false piston to the auxiliary pressure chamber at the opposite side, and a release valve for venting fluid under pressure from the respective auxiliary pressure chamber operative by said power piston upon substantial contact with the respective false piston.

3. A fluid motor comprising a casing having a bore, a double acting power piston slidably mounted in said bore, two false pistons slidably mounted in said bore, one at either side of said power piston, and cooperative with said power. piston to form two main pressure chambers one at either side of said power piston, an auxiliary pressure chamber at the opposite side of each false piston, means acting on each false piston urging it toward said power piston to a position providing for limited movement of said power piston between the two false pistons, means for conveying fluid under pressure to the main pressure chamber at either side of said power piston and for at the same time venting the main pressure chamber at the opposite side, one-way flow. means operative upon supply of fluid under pressure to either one of said main pressure chambers to charge the auxiliary pressure chamber at the opposite side of the respective false piston with fluid at substantially the same pressure, and means operative by said power piston upon substantial contact with either false piston to release fluid under pressure from the auxiliary chamber at the opposite side of the respective false piston.

4. A fluid motor comprising a casing having a bore, a double acting power piston slidably mounted in said bore, two false pistons slidably mounted in said bore, one at either side of said power piston, and cooperative with said power piston to form two main pressure chambers one at either side of said power piston, an auxiliary pressure chamber at the opposite side of each false piston, means acting on each false piston urging it toward said power piston to a position providing for limited movement of said power piston between the two false pistons, means for conveying fluid under pressure to the main pressure chamber at either side of said power piston and for at the same time venting the main pressure chamber at the opposite side, a communication through each false piston providing for flow of fluid under pressure from the main pressure chamber at one side of the respective false piston 'to the auxiliary pressure chamber at the oppo- -site side, a check valve in said communication for preventing reverse flow of fluid under pressure therethrough, another communication in each false piston for releasing fluid under pressure from the respective auxiliary pressure chamher, a release valve for closing the last named communication, and means operative by said power'piston upon substantial contactwith either pressure chamber at the opposite side of each false piston, means acting" on each false piston urging it toward said power piston to a posiition providing for limited movement of said power piston between the two false pistons, means for conveying fluid under pressure to the main" pressure chamber at either side of said power piston and for at the same time venting the main pressure chamber atthe opposite side, a communication through each false piston providing for flow of fluid under pressure from the main pressure chamber at one side of the'respective false piston to the auxiliary pressure chamher at the'opposite side, a check valve in said communication for preventing reverse flow of fluid under pressure therethrough another communication in each false piston for opening the respective auxiliary chamber to atmosphere, a release valve in each false piston normallyclos- 'ing the last named communication, and means operative by said power piston upon substantial to open the remounted in said bore, two false pistons slidabl V mounted in said bore. one at either side of said power piston, and cooperative with said power piston to provide two main pressure chambers, one at either side of said power piston, a rod projecting from said power piston having two axially extending passageways, one open to each of said pressure chambers, through which fluid under pressure is adapted to be supplied to either of-said chambers and atthe same time released from the other chamber, an auxiliary pressure chamber at the opposite side of each false piston, a rod associated with each false piston, a stop associated with the rod of each false piston arranged to cooperate with said casing to limit movement of the false pistons in the direction of said power piston, a spring acting on each false piston urging it in the direction of said power piston, one-way flow means providing for charging each auxiliary chamber with fluid at substantially the pressure provided in the main pressure chamber at the opposite side of the respective false piston, a communication in each false piston and the respective rod for opening the respective auxiliary chamber to atmosphere, and a valve normally closing the last named communication and operative by said main piston upon substantial contact with the respective false piston to open the last named communication.

7. A fluid motor comprising a casing having 18 a bore, a double acting power piston slidably mounted in said bore, two false pistons slidably mounted in said bore, one at either side of said power piston, and cooperative with said power piston to provide two main pressure chambers, one at either side of said power piston, a rod projecting from said power piston having two axially extending passageways, one open to each of said pressure chambers, valve means carried by said rod operative to supply fluid under pressure to either of said passageways while at, the same time venting fluid under presure from the other passagewa an auxiliary pressure chamber at the opposite side of each false piston, a rod associated with each false piston, a stop associated with the rod of each false piston arranged to cooperate with said casing to limit movement of the false piston in the direction of said power piston, a spring acti32g on each false piston urging it in the direction of said power piston, one-way flow means providing for charging each auxiliary chamber with fluidat substantially the pressure provided in the main pressure chamber at the opposite side of the respective false piston, a communication in each false piston and the respective rod for opening the respective auxiliary chamber to atmosphere, and a valve normally closing the last named communication andop erative by said main piston upon substantial contact with the respective false piston to open the last named communication. s

8. A fluid motor comprising a casing having a bore, a double acting power piston slidably mounted in said bore, two false pistons slidable mounted in said bore, one at either side of said power piston, and cooperative with said power piston to provide two main pressure chambers, one at either side of said power piston, a rod projecting from said power piston having two axially extending passageways, one open to each of said pressure chambers, valve means carried by said rod for controlling supply and release of fluid under pressure to and from said passageways, means operative manually relative to said rod to actuate said valve means to selectively supply fluid under pressure to either one of said passageways and at the same time to release fluid under pressure from the other passageway, means operative by said rod in response to movement thereof relative to said manually operative means to actuate said valve means to bottle up fluid under pressure in each of said passageways, an auxiliary pressure chamber at the opposite side of each false piston, a rod asscciated with each false piston, a stop associated with the rod of each false piston arranged to cooperate with said casing to limit movement of the false piston in the direction of said power piston, a spring acting on each false piston urging it in the direction of said power piston, oneway flow means providing for charging each auxiliary chamber with fluid at substantially the pressure provided in the main pressure cham-- her at the opposite side of the respective false piston, a communication in each false piston and the respective rod for opening the respective auxiliary chamber to atmosphere, and a valve normally closing the last named communication and operative by said main piston upon substantial contact with the respective false piston to open the last named communication.

9. In combination, a pivoted rockable lever, manual means for rocking said lever, and a booster power cylinder operable also to rock said lever upon the resistance to movement of said lever exceeding a chosen valve, said power cylinder comprising, a casing having a bore, two

pistons disposed in said bore, means for limiting the movement of each of said pistons in the direction of the other, a third piston slidably disposed in'said bore between said two pistons, a piston rod secured at one end to said third piston and at the other end exteriorly of said casing to said rockable lever, a counterbore in said other end of said piston rod, a sleeve slid- ,ablydisposed in said counter bore and having an operative connection with said manual means,

a first self-lapping valve means carried in said sleeve and operative by said manual means upon the resistance to rocking of said lever in one direction exceeding said chosen value to supply fluid under pressure to one face and to release fluid under pressure from the opposite face of to said opposite face and to release fluid under pressure from said one face of said third piston, means including a check valve carried by each .pfsaid twopistons operable to establish a one- "Way flow fluid pressure communication through ea ch of said two pistons upon supply of fluid ,under pressure to that face of said third piston adjacent the respective one of said two pistons lease of fluid under pressure from that face of said third piston adjacent the respective one of said two pistons, and means operative upon said third piston substantially engaging either one of said two pistons to release fluid under pressure from the opposite side of the last named piston .to permit movement then of said third piston.

10, In combination, a pivoted rockable lever, a manual means for rocking said lever, a source of fluid under pressure, and a power cylinder for rocking said lever, said power cylinder comprisinga first piston having an operative connection with said lever, a first self-lapping valve means operative by said manual means, upon the resistance to rocking of said lever in one direction exceeding a chosen value, to supply fluid under pressure from said source to one face of said first piston to cause rocking of said lever in one direction in response'to movement of said first piston,

a second self-lapping valve means operative by said manual means, upon the resistance to rock- 'ing of said leverin the opposite direction exceeding said chosen valve, to supply fluid under pressure from said source to the opposite face of said first piston to cause rocking of said lever in the opposite direction in response to movement of said first piston, a second and a third piston disposed respectively on opposite sides of said first 20 piston to form two pressure chambers on each side of said first piston, means for limiting movement of said second and third pistons in the direction of each other to provide a chosen length of stroke of said first piston without engaging either of said two pistons, valve means carried by each of said second and third pistons to eifect charging the respective chambers on that side of said second and third pistons opposite said first piston with fluid at substantially the maximum pressure effective on the adjacent face of said first piston, and exhaust valve means carried by each of'said second and third pistons and operable by said first piston upon the stroke of said first piston in one or the other direction exceeding said chosen length to release fluid under pressure from the respective chamber on that side of the respective second or third piston opposite said first piston.

11. In combination, a rockable lever, a fluid motor comprising a casing having a bore, a first piston slidably disposed in said bore and having a driving connection with said lever, two pistons disposed in said bore one on each side of said first piston and each cooperating with said first piston to form a first pressure chamber at each side of said first piston, said two pistons cooperating with said casing to form a second pressure chamber at the opposite side of each of said two pistons, a first valve means operative to simultaneously supply fluid under pressure to one of said first chambers and to release fluid under pressure from the other of said first chambers to move said first piston to rocksaid lever, one-way flow valve means carried by each of said two pistons and effective to cause charging of 'the second chamber at one side of the respective one of said two pistons upon supply of fluid under pressure to the opposite side, and exhaust valve means carried by each of said two pistons and operative by said first piston upon substantial engagement of said first piston with the respective one of said two pistons to release fluid under'pressure from said second chamber at the opposite side of the last named piston.

r ARTHUR J. BENT.

REFERENCES CITED The following references are of record in the 

